The present invention relates to an imaging tube which can favorably be used to amplify and observe a diminished-light image, to a streaking tube which can favorably be used to analyze the light intensity distributions of light sources with elapsing of time, and to a method of fabricating these types of imaging and streaking tubes.
The configuration of the conventional imaging tube and the problems to be solved in accordance wih the present invention will be described in relation to FIG. 1.
FIG. 1 shows a cross-sectional view of the conventional imaging tube together with the interrelation between the photoelectric layer and optical image.
One end of a vacuum envelope 3 of the imaging tube constitutes an incident window 1 upon which the optical image to be analyzed can be incident, and another end constitutes a light emitting window 2 from which the processed optical image can be emitted. Photoelectric layer 4, focusing electrode 6, aperture electrode 7, micro-channel-plate 8, and phosphor layer 9 are, in sequence, arranged in a space between incident window 1 and light emitting window 2 along the tube axis of vacuum envelope 3. A higher DC voltage is applied to focusing electrode 6 with respect to photoelectric layer 4, and another higher DC voltage to aperture electrode 7 with respect to focusing electrode 6. A DC voltage which is the same as or a little higher than that applied to aperture electrode 7 is applied to input electrode 8a of micro-channel-plate 8, and a higher DC voltage than that applied to input electrode 8a is applied to output electrode 8b of micro-channel-plate 8. A further higher DC voltage than that applied to output electrode 8b of micro-channel-plate is applied to phosphor layer 9.
We assume that optical image 4a is incident onto photoelectric layer 4 via incident window 1 in the setup not shown. Photoelectric layer 4 emits an electron image corresponding to the optical image, and the emitted electrons are accelerated and focused by focusing electrode 6. They pass through both aperture electrode 7 and micro-channel-plate 8, and arrive at phosphor layer 9 to be focused thereon.
Micro-channel-plate 8 consists of a strand of approximately 10.sup.6 fine glass tubes each having a secondary electron emitting surface of lead oxide deposited on its inner wall. Each fine glass tube, having an inner diameter of 15 .mu.m, is 0.9 mm long. The strand has a diameter of 25 mm.
The incident electrons are multiplied by the micro-channel-plate 8 and then the multiplied electrons are emitted from the micro-channel-plate 8. The multiplication factor depends on the voltage difference between input electrode 8a and output electrode 8b. When the voltage difference between input electrode 8a and output electrode 8b changes from 1.3 kV to 1.9 kV, the multiplication factor goes from 10.sup.3 to 3.times.10.sup.6.
Such an imaging tube as described above can be fabricated by the following method.
At first, a glass cylinder to form the wall of vacuum envelope 3 and one end of vacuum envelope 3 are constructed. Next, a first glass disc for forming a photoelectric layer on which the optical image is incident, and the other end of vacuum envelope 3 are constructed. Materials used for the envelope, i.e., a second glass disc wherein a light emitting window used to emit the optical image therefrom is formed and whereon the phosphor layer is formed, and elements used for making such electrodes as mesh electrode 5, focusing electrode 6, aperture electrode 7, and micro-channel-plate 8 are prepared. Elements used for making the electrodes are then fastened within the glass cylinder. At that time, antimony metal contained within a tungsten coil to form an evaporation source of antimony is located against the photoelectric layer substrate.
Phosphor materials are coated on one surface of the second glass disc. First and second glass discs are located at the appropriate ends of the glass cylinder, and then the resulting envelope is exhausted to obtain a vacuum.
A branching tube is fastened to the side wall of the sealed envelope and an alkaline metal source is housed in this branching tube. Air is then exhausted from the sealed envelope via the exhausting tube attached thereto.
A current is applied to flow through the tungsten coil so that antimony metal is deposited onto the photoelectric layer substrate. Alkali metal is gradually fed from the branching tube into the envelope, while the sensitivity of the photoelectric layer is being monitored, until the maximum sensitivity can be obtained. Thereafter, the branching tube is cut off. Then, the exhausting tube is also cut away to complete the imaging tube.
It can easily be understood from the description of the fabrication method that a small amount of alkali metal necessarily adheres to each electrode while alkali metal is being fed to the sealed envelope.
When an imaging tube fabricated in accordance with this process is operated, the phosphor layer sometimes emits light due to a decrease in the work function by the alkali metal when no light is incident upon the photoelectric layer.
When a high voltage is applied to micro-channel-plate 8, this mode of light emission is especially enhanced.
This mode of light emission causes the S/N ratio to decrease affecting the background noise for the image, and it makes the dynamic range low.
The inventors of the present invention found that the phosphor layer emitted light without any incident light when a voltage was applied only to the phosphor layer of the micro-channel-plate unless voltages were applied to the imaging section consisting of a photoelectric layer, a focusing electrode, and an aperture electrode. They also found that the objectionable light emission was caused by existence of the micro-channel-plate. Furthermore, they found that the background sensitivity was not increased when a set of voltage was applied to the respective electrodes after the envelope was exhausted and sealed for making a tube of the same dimensions providing no photoelectric alkali layer. The above phenomena suggests that generated electrons increase the background sensitivity due to the following reasons:
Alkali metal adheres to the inner surface of the micro-channel-plate which multiplies secondary electrons, while the photoelectric layer is being formed, and it decreases the work function of electrons at the surface. When a voltage is applied to the micro-channel-plate during operation, high electric fields are locally generated at microscopic locations of non-uniform areas on the inner surface thereof. Interaction of both the low work function and high electric field causes the inner surface of the micro-channel-plate to emit electrons.
Electrons generated due to field emission are multiplied by the micro-channel-plate and incident upon the phosphor layer to cause the unwanted background sensitivity to increase.
The streaking tube can convert the incident light pulse with a duration of 1 ns into a length on the order of several tens of millimeters on the phosphor layer, and it has an excellent timing resolution of 2 pico seconds or less. The streaking tube is thus widely used for analyzing the waveforms of laser pulses.
Next, the streaking tube in accordance with the present invention will be described hereafter.
The configuration of the conventional streaking tube and the problems to be solved in accordance with the present invention will briefly be described in relation to FIG. 2.
FIG. 2 shows a cross-sectional view of the conventional streaking tube together with the interrelation between the photoelectric layer and optical image.
One end of a vacuum envelope 3 of the streaking tube constitutes an incident window 1 upon which the optical image to be analyzed can be incident, and another end constitutes a light emitting window 2 from which the processed optical image can be emitted. Photoelectric layer 4, mesh electrode 5, focusing electrode 6, aperture electrode 7, deflection electrode 108, and phosphor layer 9 are, in sequence, arranged in a space between incident window 1 and light emitting window 2 along the tube axis of vacuum envelope 3. A higher DC voltage is applied to mesh electrode 5 with respect to photoelectric layer 4, another higher DC voltage to focusing electrode 6 with respect to mesh electrode 5, and a further higher DC voltage to aperture electrode 7 with respect to focusing electrode 6. A DC voltage which is the same as or a little higher than that applied to aperture electrode 7 is applied to phosphor layer 9.
We assume that linear optical image 4a which lies in the center of the photoelectric layer 4 is incident onto photoelectric layer 4 via incident window 1 in the setup not shown. Photoelectric layer 4 emits an electron image corresponding to the optical image, and the emitted electrons are accelerated by mesh electrode 5 and focused by focusing electrode 6. They pass through both aperture electrode 7 and deflection electrode 108 and arrives at phoshpor layer 9 to be focused thereon.
While the linear electronic image is passing through a gap within deflection electrode 108, a deflection voltage is applied to the deflection electrode 108. The electric field caused by this deflection voltage is normal to both the tube axis and linear electronic image. (Note that the electric field is normal to the plane of the drawing in FIG. 2.) The field strength is proportional to the deflection voltage. The electron beam on phosphor layer 9 travels normal to the linear electronic image when scanned. A series of sequential linear optical images are arranged onto photoelectric layer 4 in a direction perpendicular to the linear images, and thus a streaking image is formed. Brightness changes in the direction that a series of linear optical images are arranged on that scanning is being carried out indicates a change in intensity of the optical image incident on phosphor layer 4.
Such a streaking tube as described above can be fabricated by the following method:
At first, a glass cylinder to form the wall of vacuum envelope 3 and one end of vacuum envelope 3 are constructed. Next, a first glass disc for forming a photoelectric layer on which the optical image is incident, and the other end of vacuum envelope 3 are constructed. Materials used for the envelope, i.e., a second glass disc wherein a light emitting window used to emit the optical image therefrom is formed and whereon the phosphor layer is formed, and elements used for making such electrodes as mesh electrode 5, focusing electrode 6, aperture electrode 7, and deflection electrode 108 are prepared. Elements used to make the electrodes are then fastened within the glass cylinder. At that time, antimony metal contained within a tungsten coil to form an evaporation source of antimony is located against the photoelectric layer substrate.
Phosphor materials are coated on one surface of the second glass disc. First and second glass discs are located at the appropriate ends of the glass cylinder, and then the resulting envelope is exhausted to obtain a vacuum.
A branching tube is then fastened to the side wall of the sealed envelope and an alkaline metal source is housed in this branching tube. Air is exhausted from the sealed envelope via the exhausting tube attached thereto.
A current is applied to flow through the tungsten coil so that antimony metal is deposited onto the photoelectric layer substrate. Alkali metal is gradually fed from the branching tube to the envelope, while the sensitivity of the photoelectric layer is being monitored, until the maximum sensitivity is obtained. Thereafter, the branching tube is cut off. Thereafter, the exhausting tube is cut away to complete the streaking tube.
It can easily be understood from the description of the fabrication method that a small amount of alkali metal necessarily adheres to each electrode while alkali metal is being fed to the sealed envelope.
When a streaking tube fabricated in accordance with this process is operated, the phosphor layer sometimes emits light due to a decrease in the work function by the alkali metal when no light is incident upon the photoelectric layer.
When an RF voltage is repetitively applied to deflection electrode 108, this mode of light emission is especially enhanced.
This mode of light emission causes the S/N ratio to decrease affecting the background noise for the streaking image, and it makes the dynamic range low.
The inventors of the present invention studied the photoelectrons, on the photoelectric layer, which were generated due to light emitted by excitation or ionization of gaseous molecules or atoms which had collided with electrons, or by collision of electrons or ions with the sealed envelope, and they found that the main reason for their generation was caused by the effect of the deflection electrode on the dynamic range.
We found that, unless a voltage was applied across a pair of deflection electrodes although a high DC voltage was applied across photoelectric layer 4 and aperture electrode 7, light emission occurring in phosphor layer 9 was diminished in intensity while enhanced by the repetitive sweep voltage applied across the deflection electrode.